1. Field of the Invention
The present invention relates to a new method for rapidly quenching large sections of precipitation hardenable alloys.
2. Background
A precipitation hardenable (or xe2x80x9cage hardenablexe2x80x9d) alloy is an alloy which, when heated at a temperature below its solvus temperature, nucleates and grows a precipitate of alloy components. Precipitation hardening normally causes a noticeable increase in alloy hardness as well as beneficial enhancement of other alloy property combinations including, for example, strength, ductility and electrical conductivity.
Industrially, precipitation hardening is accomplished by heating the alloy at a fairly narrow temperature range roughly midway between the solvus temperature and room temperature for 0.5 to 20 hours. Precipitation hardening temperatures approaching the solvus temperature are usually avoided, since it is difficult to control the results obtained at these higher temperatures and the nature of the precipitates changes significantly. Precipitation hardening at less than a minimum practical hardening temperature at which precipitation hardening is too slow to be commercially feasible is also avoided.
Precipitation hardening will not normally occur unless the ingredients of the alloy are distributed fairly uniformly in the alloy mass. Therefore, precipitation hardenable alloys are normally subjected to one or more heat treatment and/or wrought processing steps, prior to precipitation hardening, to reduce the gross and/or micro-segregation of elements which inherently occurs when molten alloys solidify and to refine microstructure. Examples of such processing steps include homogenization, solution annealing, hot working and cold working.
In homogenization, the alloy is heated at a temperature below but relatively near the alloy""s solidus temperature for an extended period of time such as 4 to 12 hours, for example, and then quenched. Homogenization is normally done early in the processing regimen, normally as the first processing step after the alloy is cast. As a result of homogenization, the alloy solute elements tend to dissolve in the alloy matrix, thereby achieving a more nearly uniform distribution of ingredients. Quenching after homogenization can be rapid or slow and is most typically done by air-cooling.
Solution annealing is similar to homogenization in that the ingot is also heated near but below its solidus temperature. However, solution annealing normally presupposes that the alloy already starts with a fairly uniform element distribution, with heating being done merely to dissolve elements that may have undergone short-range segregation during cooling from a prior hot working or heat-treatment step. Furthermore, heating times are usually significantly shorter than in conventional homogenization, on the order of a few minutes to several hours or so. Section size, that is the size of the metal mass or section being heated, also plays a role in heating times because of thermal conductivity limitations.
Solution annealing also connotes that the alloy is rapidly quenched to a temperature at or near ambient normally to its lowest hardness condition. By xe2x80x9crapid quenchingxe2x80x9d is meant that the temperature of the alloy throughout its mass is reduced as rapidly as possible on a commercially feasible basis. Usually, rapid quenching is done by immersion in water, although other techniques can be used such as contact with oil, cooling gas or other material. Rapid quenching xe2x80x9cfreezesxe2x80x9d the dissolved ingredients in place, thereby preventing formation of other phases which can occur if cooling is slower.
In hot and cold working, the alloy is subjected to significant, uniform mechanical deformation to mechanically break up larger crystal grains into smaller sizes. Hot working is normally done between the alloy""s solvus and solidus temperatures, thereby allowing recrystallization of the alloy components into smaller grains upon cooling. Cold working is normally done at ambient temperature and, in any event, below precipitation hardening temperatures. Cold working can be followed by solution annealing, which also promotes recrystallization of the alloy ingredients into smaller grains.
Solution annealed, precipitation hardenable alloys in the form of large sections are difficult to produce reliably and consistently. In this context, xe2x80x9csectionxe2x80x9d means a mass of the alloy whether or not previously worked to change its size or shape. In some instances, the alloy is not fully hardenable as reflected by insufficient strength and/or hardness when the alloy is precipitation hardened. In other instances, the alloy mass suffers internal cracking during heat treatment or distortion during subsequent machining and/or use. Depending upon the particular alloy involved, these problem are observed in sections whose minimum caliper (minimum thickness dimension) is as little as 3 inches. In other alloys, these problems are not observed until the minimum caliper of the section is 8 inches or more. Thus, a xe2x80x9clargexe2x80x9d section of a precipitation hardenable alloy in the context of this case means a section whose minimum caliper is large enough so that, after conventional solution annealing using a water immersion quench, one or more of the above problems is observed.
In accordance with the present invention, it has been discovered that large sections of solution annealed, precipitation hardenable alloys which are resistant to internal cracking and distortion yet fully hardenable can be produced if, during quenching of the alloy, the temperature of the section is allowed to stabilize immediately above the solvus temperature before the section is rapidly quenched. Preferably, the temperature of the section is allowed to stabilize a second time, at the end of rapid quenching, before the section is cooled to ambient.
Thus, the present invention provides a new process for quenching a precipitation hardenable alloy in which the alloy is cooled from a solution anneal temperature down to a final quench temperature, the process comprising allowing the temperature of the alloy to stabilize at a first stabilization temperature immediately above the solvus temperature of the alloy before the alloy is rapidly quenched. Preferably, the temperature of the alloy is also allowed to stabilize a second time at a second stabilization temperature higher than the final quench temperature yet not so high that any significant phase or hardness change occurs in the alloy, before cooling to the final quench temperature.
In addition, the present invention further provides as new products, large sections of precipitation hardenable alloys which are fully hardenable and yet have a reduced tendency for internal cracking and distortion, the alloy sections being made by a heat treatment process in which the temperature of the section is allowed to stabilize immediately above the alloy""s solvus temperature before rapid quenching.